BNC 1201 - 6 1/2 Digit Digital Multimeter - Manual (Page 110)

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619 Ohms 1 mA source failed his test configures to the 1000V dc
range with the internal 10M 100:1 divider R204 connected across the
input. The 1mA ohms current source is connected. The compliance limit
of the current source is measured. A 20ms ADC measurement is
performed and the result is checked against a limit of 0.12V  0.01V.
620 AC rms zero failed This test configures for the 100mV ac range
with the ac input grounded. The internal residual noise of the ac section
is measured and checked against a limit of -10mV to 70mV at the output
of the rms-to-dc converter.
621 AC rms full scale failed This test configures for the 100mV ac
range. The 1mA ohms current source is switched on to charge the ac
input capacitor C614. This produces a pulse on the output of the
rms-to-dc converter which is sampled 100ms after the current is applied.
A 20ms A/D measurement is performed and checked against a limit of 2V
to 13V into the ADC.
624 Unable to sense line frequency The supplied voltage AC2 is
routed through a comparator U4 to generate a logic input signal. This
test checks that the logic input from U4 to panel MCU U3 is toggling. If no
logic input is detected, the instrument will assume 50Hz line operation
for all future measurements.